The present invention relates to communication systems, communication methods and a method of communicating data within a DECT communication system.
Cordless communication systems typically include a handset station coupled via a radio connection with a base station. The base station is usually connected by wire to a telecommunications network, such as a traditional Public Switched Telephone Network (PSTN) or an Integrated Services Digital Network (ISDN).
Ease and convenience of use has expanded applications and popularity of cordless communication systems. Responsive to such increased popularity and applications, new standards have been introduced including digital communication standards. The development of new cordless standards based upon digital technology are useable in a broad spectrum of applications. Exemplary cordless applications include wireless Private Automatic Branch Exchange (PABX), wireless Local Area Network (LAN), Telepoint, and Radio Local Loop. Exemplary cordless standards include Digital Enhanced Cordless Telecommunications (DECT), GSM, PHS, AMPS, IS54 or IS95. One exemplary benefit of digital cordless telephones is the provision of a valid alternative to cellular phones in densely populated areas.
The Digital Enhanced Cordless Telecommunications standard is a cordless standard defined as a Multicarrier (MC), Time Division Multiple Access (TDMA)/Time Duplex Division (TDD) system. The Digital Enhanced Cordless Telecommunications standard is discussed generally in Jerry D. Gibson, The Communications Handbook, (CRC Press 1997), incorporated herein by reference.
Time is divided in the DECT standard into frames of 10 ms. Individual frames are divided into twenty-four full slots. The standard also allows for slots of differing lengths including half slots of data (half the length of a full slot) and double slots of data (double the length of a full slot). Provision of different slot types within DECT allows use with different services having different data rates. For example, in DECT/ISDN applications, both full and double slots are utilized to provide ISDN service via a DECT network.
To provide support for multiple channels, a standard DECT base station compresses and transmits 10 ms of speech during one full slot. According to the DECT standard, 10 ms of speech is sent over the radio in 416 xcexcs. Individual active connections make use of two slots, one for receiving and one for transmitting. For example, if plural slots in a DECT frame are numbered from zero to twenty-three, the first twelve slots (0-11) are used for transmission from the base station to the handset and the remaining slots are used for handset to base station transmissions. A base station transmitting to a given handset in slot N receives from this handset in slot N plus twelve, or in other words, half a frame later. Accordingly, a DECT base station operating in accordance with the standard is able to support up to twelve active voice connections simultaneously.
The total number of bits within a conventional DECT slot is 480. With twenty-four slots and a 10 ms frame, a gross bit rate of 1.152 Mbits/second is provided. Once the DECT slot has been formatted, it is transmitted using one of ten radio frequencies specified within the DECT standard. For example, the frequency band assigned to DECT in Europe is between 1,880 and 1,900 MHz, with a spacing of 1.728 kHz between adjacent frequencies. The transmission frequency for each channel is chosen dynamically based upon a Radio Signal Strength Indication (RSSI). Individual active slots in a DECT frame may be transmitted and received on any of the ten frequencies.
Enabling communication of slots having a plurality of lengths increases the flexibility and applications of the communication system. The present invention enables communication of slots having different lengths while alleviating timing problems associated with such communications.